If you can't a good grip with a strap wrench, get a piece of flat rubber and wrap it around the feedneck to protect it and then put the feedneck in a vice and twist the body (I used a mouse pad). Rubber is better than putting it between two piece of wood as it grips/protects better. I had to do this to my last marker as the strap wrench kept slipping.

If it's a threaded/removable feedneck, when you look at the bottom of the feedneck there should be a seam where the outer sleeve meets the stalk it's screwed on to, if it's not there; you have a non threaded feedneck and will need to do as the guide I posted suggests.

Bad example of the seam (dang camera focused on my leg rather than the feedneck):